1. Field of the Invention
This invention relates to an image-forming apparatus that transfers a developer to a recording medium to form an image, and more particularly to an image-forming apparatus that employs a scorotron type of charger.
2. Description of the Related Art
Image-forming apparatuses are known in the prior art which comprise an electrostatic latent image carrier such as a photosensitive drum on the surface of which are formed electrostatic latent images, a scorotron type of charger that charges the surface of the electrostatic latent image carrier uniformly by corona discharge, electrostatic latent image forming means for forming the electrostatic latent images on the surface of the electrostatic latent image carrier after charging by the charger, and development means for developing, with a developer such as toner, the electrostatic latent images formed by the electrostatic latent image forming means. With this type of image-forming apparatus, electrostatic latent images are formed by the electrostatic latent image forming means on the surface of the electrostatic latent image carrier that is charged uniformly by the charger, the electrostatic latent images are developed by developer charged by friction-charging by the development means, and then developer is transferred to a recording medium to form images.
The scorotron type of charger here described excels in uniform charging. The electrostatic latent image carrier is charged without contact, so that it exhibits less deterioration in toner adhesion, and excels in durability, as compared with a charger using contact charging. Also, with so-called electrophotographic image-forming apparatuses that form images by transferring a developer such as toner to a recording medium, maintenance can be simplified by using a cartridge constituted of i) toner that is consumed and ii) the constituting components such as the electrostatic latent image carrier and development means where the electrostatic latent image carrier exhibits deterioration faster than the other constituting components, so that the cartridge can readily be replaced as one unit. On the other hand, there is keen demand to reduce the number of replacement components as far as possible in the interest of environmental protection. The idea has thus been conceived of constitution the scorotron type of charger, which excels in durability, separately from the replacement components unified in the cartridge, so that it can be used repeatedly.
However, the scorotron type of chargers also deteriorate, for the reasons noted next, and from time to time must be replaced inevitably. For that reason they are in some cases incorporated into the cartridge as one of the replacement components. More specifically, in recent years, instead of the two-component developer systems containing toner and carrier, single-component developer systems containing no carrier have come into wide use in view of the ease of maintaining image-forming apparatuses and making them compact. With single-component developer systems, a substance corresponding to the carrier is contained in the toner, and, when the toner is moved using magnetic force, the toner is made to contain magnetic materials. However, the magnetic materials are opaque, so that, especially with color development, it is more suitable to use a non-magnetic developer. That being so, in recent years, it is becoming increasingly common to use a non-magnetic single-component developer in electrophotographic image-forming apparatuses.
With non-magnetic single-component developers, however, various additives such as silica are added to the toner to secure its fluidity. Among these additives are those which peel and drop away from the toner prior to transfer to the recording medium. Such additives sometimes adhere to the discharge wires on the scoroton type of chargers. When this happens, an insulating film is formed on the surface of the discharge wires, and hence the surface of the electrostatic latent image carrier can no longer be adequately charged. In some cases, for example, discharge irregularities are caused by the film, and the images are not properly formed in places corresponding to the film. Thereupon there is no alternative but to replace the scorotron type of charger, making it very difficult to use it separately from the other replacement components or to use it repeatedly, as described above. It is possible to prevent the adherence of additives to the discharge wires by forming air currents about the discharger, but providing the necessary mechanisms therefor places enormous restrictions on the constitution of the image-forming apparatuses. That being so, improving the durability of the chargers by providing such mechanisms is not very practical.